1. Field of the Invention
The present invention relates to a case for miniature fuel cells, and more specifically to a case enabling dissipation of the heat generated by fuel cells.
2. Discussion of the Related Art
Miniature fuel cells especially aim at equipping portable electronic equipments such as computers, telephones, music readers, etc.
FIG. 1 shows an example of an integrated fuel cell formed by using microelectronics techniques. This cell is formed on a silicon wafer 1 coated with a first thin insulating layer 2 and with a second thicker insulating layer 3. An opening is formed in a portion of insulating layer 3. A stack of a support 4, a catalyst layer 5, an electrolyte 6, and a second catalyst layer 7 is located in this opening. This layer assembly forms the active stack of the fuel cell. An electrode 10 placed on first insulating layer 2 enables taking a contact on the lower surface of the fuel cell, on support 4. An opening 11 in second insulating layer 3 provides access to electrode 10. An upper electrode 12 enables taking a contact on upper catalyst layer 7. Electrodes 10 and 12 are provided with openings, and channels 13 are formed in silicon wafer 1 opposite to the openings in the lower surface metallization. Lower electrode 10 and upper electrode 12 respectively form an anode collector and a cathode collector.
Electrolyte 6 is, for example, a polymer acid such as Nafion in solid form and catalyst layers 5 and 7 for example are carbon and platinum based layers. This is an example of embodiment only. Various types of fuel cells that can be formed as illustrated in FIG. 1 are known in the art.
To operate the fuel cell, hydrogen is injected in the direction indicated by arrow H2 on the lower surface side and air (oxygen carrier) is injected on the upper surface side. The hydrogen is “broken down” at the level of catalyst layer 5 to form, on the one hand, H+ protons which move towards electrolyte 6 and, on the other hand, electrons which move towards the outside of the cell through anode collector 10. The H+ protons cross electrolyte 6 to join catalyst layer 7 where they recombine with oxygen, coming from outside of the cell through the cathode collector, and with electrons. In known fashion, with such a structure, a positive voltage is obtained on cathode collector 12 (oxygen side) and a negative voltage is obtained on anode collector 10 (hydrogen side).
It should be underlined that FIG. 1 is not to scale. In particular, silicon wafer 1 typically has a thickness on the order of from 250 to 700 μm while the active stack of layers 4 to 7 typically has a thickness on the order of from 30 to 50 μm.
A conventional fuel cell is formed of a large number of adjacent cells of the type shown in FIG. 1, generally several hundreds, which are properly connected.
Such fuel cells cannot be placed in conventional integrated circuit housings since one of their surfaces needs to be open towards an enclosure containing hydrogen and the other one of the surfaces needs to be exposed to the ambient air. Further, these cells generate heat and, on the side of the surface exposed to oxygen, humidity.